Quantum information processing uses quantum mechanical phenomena, such as energy quantization, superposition, and entanglement, to encode and process information in a way not utilized by conventional information processing. For example, certain computational problems may be solved more efficiently using quantum computation rather than conventional classical computation. However, to become a viable computational option, it may be necessary to precisely control a large number of quantum bits, known as “qubits,” and to control interactions between these qubits. In particular, qubits may ideally have long coherence times, be able to be individually manipulated, be able to interact with one or more other qubits to implement multi-qubit gates, be able to be initialized and measured efficiently, and be scalable so that a quantum computer can include large numbers of qubits.
A qubit may be formed from any physical quantum mechanical system with at least two orthogonal states. The two states of the system used to encode information are referred to as the “computational basis.” For example, photon polarization, electron spin, and nuclear spin are all two-level systems that may encode information and may therefore each be used as a qubit for quantum information processing. Different physical implementations of qubits have different advantages and disadvantages. For example, photon polarization benefits from long coherence times and simple single qubit manipulation, but suffers from the inability to create simple multi-qubit gates.
Different types of superconducting qubits using Josephson junctions have been proposed, including “phase qubits,” where the computational basis is the quantized energy states of Cooper pairs in a Josephson Junction; “flux qubits,” where the computational basis is the direction of circulating current flow in a superconducting loop; and “charge qubits,” where the computational basis is the presence or absence of a Cooper pair on a superconducting island. Superconducting qubits are an advantageous choice of qubit because the coupling between two qubits is strong, making two-qubit gates relatively simple to implement, and superconducting qubits are scalable because they are mesoscopic components that may be formed using conventional electronic circuitry techniques.